They never have been used as media server. Belive we did binary copy or server push installation to make them as server instead of client mistakenly. But we do backup them regulary according to our policies.

My experience has been that machines show up as Media Servers in Host Properties when Storage Units have been created.
Please check if Storage Units exist for these clients.
Use GUI on Master or this command:
bpstulist -L

If STUs are listed for these clients, best to use decommision process as indicated above.

Supporting Storage Foundation and VCS on Unix and Windows as well as NetBackup on Unix and WindowsHandy NBU Links